Resistive memory is nonvolatile memory and is characterized by that it has a data processing rate of volatile memory and retains the stored data even when not powered.
For instance, when voltage and current are supplied to phase-change random access memory (PRAM) in which a phase-change layer is inserted between a top electrode and a bottom electrode, heat is retained in the phase-change layer, which changes electrical conduction state according to the change in resistance. AgInSbTe is a material that is usually used for the phase-change layer. The phase-change layer is usually formed using a compound having a chalcogen element (e.g., S, Se or Te) as a main component, and more particularly, using an alloy (e.g., Ge2Sb2Te5) of germanium (Ge), antimony (Sb) and tellurium (Te).
The PRAM includes a phase-change resistor. When a current lower than a threshold value flows in the phase-change resistor, the phase-change layer reaches an appropriate temperature at which the phase-change layer is crystallized. Accordingly, the phase-change layer transforms into a crystalline state, i.e., a low resistance state.
In contrast, when a current higher than the threshold value flows in the phase-change resistor, the phase-change layer transforms into an amorphous state. In other words, data can be stored in a nonvolatile manner by using the resistance state which changes with the magnitude of current.
Meanwhile, a cell array of the resistive memory device includes a unit cell at each of intersections between a plurality of bit lines and a plurality of word lines. The unit cell can be represented with a resistor and a diode in a circuit. The current flowing in the resistive memory device may vary with processes or a path through which the current reaches each cell. As the current has a variation, a resistance value changes and a read operation and a write operation can be performed distinguishably. Accordingly, it may be desirable that there is a big difference between the highest resistance value and the lowest resistance value. It may be required to calculate a resistance value of a bit line in order to recognize how much current is required with respect to the distribution of cells to enable the read operation on a cell to be clearly distinguished from the write operation on the cell.